For the most part, the tartaric acid used in the industry is produced from byproducts containing potassium hydrogentartrate, which are obtained during wine preparation, such as tartar, wine yeast, marc, distillation residues, calcium tartrate, etc. In the better known processes, potassium hydrogentartrate is obtained by precipitation; the raw materials to be processed either are attacked with an acid and potassium hydrogentartrate is precipitated from the solution obtained by means of a basic agent or the raw materials are treated with a basic agent and the potassium hydrogentartrate is precipitated by adding an acid.
In the process described in DE-C-264005, the raw materials to be processed are roasted and treated with basic and acid agents such that both a solution of a neutral tartrate and a solution of a free tartaric acid are obtained. By mixing the two solutions, the tartaric acid is precipitated as potassium hydrogentartrate.
From FR-A-2646421, there is known a process for producing tartaric acid, in which the bitartrates contained in the raw materials are neutralized to obtain soluble tartrates. The aqueous solution of the tartrates is subjected to an electrodialysis, in which the ion exchange is effected on semipermeable membranes, for the purpose of producing a tartaric acid solution. The construction and operation of such an electrodialysis plant requires quite a considerable effort. In addition, the tartrate solutions subjected to the ion exchange must be free of organic substances, as the same would otherwise be decomposed during electrodialysis to form ammonia.
DE-A-19819884, is concerned with a process of producing tartaric acid from raw materials whose dry matter consists of at least 5.0 wt % potassium hydrogentartrate, in which potassium hydrogentartrate is first converted to dipotassium tartrate (K2H4C4O6) by stirring with aqueous potassium hydroxide solution. The aqueous solution formed is stirred at a pH-value of 2 to 5 upon removal of impurities by filtration in a precipitation stage with added acid. The added acid is preferably tartaric acid recirculated from the process. From the formed suspension containing potassium hydrogentartrate crystals, the potassium hydrogentartrate crystals are separated, washed with water, producing an aqueous solution saturated with potassium hydrogentartrate of at least 80 wt %. Potassium is removed from this solution, and from the aqueous tartaric acid solution formed, water is at least partly removed. During the conversion of potassium hydrogentartrate by means of aqueous potassium hydroxide solution to obtain dipotassium tartrate, the organic compounds contained therein, such as phenolates, sugar polymers, slimy substances and other cell ingredients, are disadvantageously dissolved, which greatly impedes filtration.